1. Technical Field
The present invention relates to an image processing device. Specifically, the present invention relates to an ink jet printer for outputting color ink in particulate form (ink dots) onto a print media to thereby form desired characters and images.
2. Related Art
In line with widespread consumer use of personal computers, digital cameras, and so on, ink jet printers are also now widely used by consumers, as well as in offices. The popularity of such printers is due to their ability to provide high quality color printing at a low cost.
Generally, ink jet printers have a carriage in which ink cartridges with integral printing heads are provided. In operation, the carriage of the printer is caused to scan in a direction orthogonal to that in which a sheet for printing is fed through the printer (chassis), and ink is ejected in particulate form from the printing heads onto a print media to thereby form thereon desired characters and images. Ink jet printers are capable of forming color images as well as monochrome images by use of a carriage having color ink cartridges (for example, black (K), yellow (Y), magenta (M), and cyan (C)), the cartridges being provided with an integral printing head. To obtain higher quality color images, recently some ink jet printers have been developed having six, seven, or eight color ink cartridges (for example, light-cyan and light-magenta as well as YMCK).
An ink jet printer that carries out printing by scanning of its cartridge carriage in a direction orthogonal to a print media feeding direction is referred to as a “multi-pass printer”. In using such a printer, to obtain an image that covers an entire surface of a print media, it is necessary for the carriage of the printer to scan orthogonal to the media tens of to more than a hundred times. Consequently, such printers suffer from a disadvantage in that they take a long time to print an image as compared to, for example, laser printers using electrophotography.
To attain high speed printing, there has been developed an ink jet printer having a long printing head and no carriage. This type of ink jet printer is referred to as a “line head printer”. In using a line head printer, there is no necessity to move a printing head in a direction orthogonal to that of feeding of print media. A line head, printer carries out so-called “single-pass printing”; and is able to produce prints at a rate that is as fast as that of a laser printer.
The printing head of a single-pass printer is provided with micro nozzles having a diameter of 10-70 micrometers. The nozzles are provided in the printing head at regular intervals; and their layout may be such as is capable of printing a plurality of lines. It is preferable that the sizes of ink particles ejected from each nozzle remain constant for all nozzles. However, in practice, a size of ink particles ejected from each nozzle may be subject to slight deviations. Such deviations cause inconsistencies in a density of ink printed on a print medium.
In addition, in a case that an output direction of a nozzle of such a printing head deviates from its ideal direction because of a fabrication error, or in a case that a position of the nozzle deviates from its ideal position because of a fabrication error, an ink dot formed on a paper by the nozzle will also deviate from its ideal position. This phenomenon is referred to as “splash bending”, and causes print errors. Splash bending causes a deterioration in print quality. Specifically, intervals between adjacent ink dots become non-uniform by splash bending. Thus, at a part of a printed image in which two ink dots are printed at an interval that is longer than the ideal interval, a light band is likely to appear; and at a part of printed image in which two ink dots are printed at an interval that is shorter than the ideal interval, a dark band is likely to appear. This phenomenon is referred to as “banding”.
For example, JP-A-11-058732 discloses a method of compensating for a deviation in a size of an ink dot. In JP-A-11-058732, each nozzle has its own gain control circuit. The gain control circuit controls a gain of each nozzle so that the size of ink dots formed by the nozzles is constant and equal to the required density.
To prevent banding, two approaches are available. One approach is to improve hardware such as a printing head or a carriage. The other approach is to improve software for processing image data. For example, Japanese Patents No. 3176182, 3176183, 3176184, and 3478573, JP-A-2001-018374, and JP-2001-150701 disclose software approaches to prevent banding. According to Japanese Patents No. 3176182, 3176183, 3176184, and 3478573, the printing head is repeatedly moved over a predetermined area. Thus, the characteristics of the nozzles are inconspicuous because a plurality of nozzles eject ink dots onto approximately the same point on a print media. JP-A-2001-018374 and JP-2001-150701 disclose ink jet printers having a plurality of printing heads. The ink jet printers print an image at a predetermined area by using the plurality of printing heads, each of which has different output characteristics, which prevents deviations in characteristics from becoming conspicuous in printing.
According to a method disclosed in JP-A-11-058732, it is necessary for each nozzle to have a dedicated gain control circuit. This results in a disadvantage in that the printer of JP-A-11-058732 is expensive to fabricate and complex. Furthermore, JP-A-11-058732 is subject to another problem in that the printer has to control all of the gain control circuits, and therefore has a high control-load.
Furthermore, Japanese Patents No. 3176182, 3176183, 3176184, and 3478573, JP-A-2001-018374, and JP-2001-150701 cannot be applied to a line head printer, because the printing head of a line head printer cannot move in a direction orthogonal to the print media feeding direction. It is desired to develop image processing for reducing banding, which is also capable of being applied to a line head printer.